please go to the buttons > on the left side on the top you will see 3 dots > click on them and choose arrange > bring forward or bring to front. try both depending on your situation
That must be a momentary thing but there is a way to make sure this never happens; Let the overlay be a part of the BG image you upload to Adalo. So you will not need to add a rectangle in Adalo.
For example, take the image you are using for the background and add the rectangle as a fill in Figma or Sketch then export the image and use it in Adalo. That the rectangle is part of the background.